One of the most surprising findings from cancer genome sequencing efforts was the identification of recurrent somatic mutations in genes encoding pre‐mRNA splicing factors in both hematologic malignancies including MDS, AML, and CLL,15, 16, 17, 18, 20, 51 and in epithelial cancers such as uveal melanoma, lung adenocarcinoma, breast cancer, and pancreatic ductal adenocarcinoma.19, 21, 22, 27, 30, 52 While mutations have been observed in a large number of spliceosomal genes, SF3B1, SRSF2, U2AF1, and ZRSR2 are the four most commonly mutated genes (Figure 1). The gene discussed is U2AF1; the disease is breast cancer.